On August 17, 2017 the LIGO and Virgo collaboration measured a gravitational wave originating from a binary neutron star merger. Within 1.7 seconds, this event was followed by a gamma ray burst (GRB), recorded by the Fermi and INTEGRAL satellites, and subsequent electromagnetic observations in the optical and radio regimes. These observations made it possible for the first time to pinpoint the source location of a gravitational wave event. The source was found to be in a galaxy 130 million light years away, known as NGC 4993. According to model predictions such a system may also accelerate cosmic rays to extreme energies, and thus emit photons and neutrinos up to GeV to EeV energies.

Ever since the existence of cosmic rays with individual energies of several Joules was established in the 1960s, speculation has raged as to whether cosmic particles of mean energy of 2 Joules are created in our Milky Way or in distant extragalactic objects.

Are you interested in brushing up your Danish? Science journalist Gunver Lystbæk Vestergård wrote a feature article on the Pierre Auger Observatory that appeared in the Danish weekly newspaper Weekendavisen – the second largest in Denmark.

Cosmic rays are mostly charged particles that reach Earth from outer space. Although they were discovered more than a century ago, the origin of the ultra-high energy ones (with an energy above 1018 eV, or 100,000 times more energetic than the particles in the LHC beam, the most powerful particle accelerator on Earth) is still a puzzle. The large area of the Pierre Auger Observatory of about 3000 km2 compensates for the low flux of such particles and allows us to search for an excess/deficit of events arriving from different directions in the celestial sphere.

The Pierre Auger Collaboration has searched for the possible presence of photons with energies exceeding 1018 eV in the flux of cosmic rays arriving at Earth. These ultra-high energy photons are produced in interactions of the charged cosmic rays with energies close to 1020 eV with the cosmic microwave background (GZK effect).